This invention relates to a speed control device for controlling the sewing speeds of various types of sewing machines, and more particularly to a speed control device for setting of sewing speeds separately according to the types of sewing machine to be used.
FIG. 1 shows the arrangement of a sewing machine driven by an electric motor, which comprises a sewing machine body 14, a sewing machine spindle 15, a pulley 16a fixedly mounted on the spindle 15, a sewing machine speed detector 17 for detecting the rotional speed of the sewing machine spindle 15, an electric motor 18 for rotating the sewing machine body 14, the rotary shaft 18 of the electric motor 18, a pulley 16b fixedly mounted on the rotary shaft 19, an endless belt 20 for transmitting the rotation of the pulley 16b to the pulley 16a, a sewing machine control unit 21, a motor speed detector for detecting the rotational speed of the motor 18, and a pedal 23 for varying the rotational speed of the motor 18.
In the sewing machine thus constructed, the pedal 23 is stepped in so that the sewing machine control unit 21 applies a signal to the motor 18 to rotate the motor 18. In response to the signal, the motor 18 starts to rotate and then the rotational force of the motor is transmitted through the pulley 16b, the endless belt 20 and the pulley 16a to the sewing machine spindle 15, so that the spindle 15 is rotated, that is, the sewing machine body 14 is actuated.
In this operation, the rotational speed of the motor 18 is detected by the motor speed detector 22 coupled to the rotary shaft 19 of the motor 18 and its information is supplied to the sewing machine control unit 21. On the other hand, the rotational speed of the sewing machine spindle 15 is detected by the sewing machine speed detector 17, and its information is supplied to the sewing machine control unit 21. The sewing machine control unit 21 controls the rotational speed of the motor 18, in a feedback mode, according to the rotational speed of the sewing machine spindle 15 thus detected so that the sewing machine body is actuated at a predetermined sewing speed.
FIG. 2 is a block diagram showing a conventional sewing machine speed control device for use in the sewing machine control unit 21. The conventional sewing machine speed control device as shown in FIG. 2 includes speed control means 1 comprising, for instance, a micro-computer for controlling the rotational speed of the sewing machine, variable resistors 2 to 5 for setting sewing speeds of the machine such as high speed, low speed, positioning speed and thread cutting speed in accordance with the operating modes of the sewing machine, which are connected between a DC control power source and ground, an analog switch 6 for selecting one of the sewing speeds determined respectively by the variable resistors 2 through 5, a sample-and-hold circuit 7 for holding for a predetermined period of time the selected sewing speed which is outputted by the analog switch, and A-D (analog-to-digial) converter 8 for converting the output analog signal of the sample-and-hold circuit 8 into a digital signal, and memory means 9 for storing data such as control constants.
In the case of controlling the sewing speed of the sewing machine body with the speed control device thus constructed, it is generally necessary to set the speed of the sewing machine body separately according to the high speed, low speed, positioning and thread cutting operations of the sewing machine body.
The operation of the speed control device as shown in FIG. 2, when the sewing machine body is operated in high speed mode, will be described hereinafter assuming that the high speed, low speed, positioning speed and thread cutting speed are set with the variable resistors 2, 3, 4 and 5, respectively.
In the high speed mode, the rotational (sewing) speed of the machine can be changed by adjusting the variable resistor 2; i.e., by changing the resistance of the resistor 2. When the resistance of the variable resistor 2 is changed, an analog signal (for instance, 0 to 5 V) corresponding to the control voltage is applied from the variable resistor to the analog switch 6. On the other hand, the analog switch 6 selects one of the input terminals and connects it to the common output terminal in response to a control signal provided by the speed control means 1 to thereby select one of the output analog signals of the variable resistors 2 through 5. When the input terminal connected to the variable resistor 2 is selected and connected to the common output terminal by the analog switch 6, the analog signal outputted from the variable resistor 2 is applied to the sample-and-hold circuit 7 as it is.
The input analog signal which changes with time is held by the sample-and-hold circuit 7 every predetermined period, and is supplied to the A-D converter 8. The analog signal inputted to the A/D converter is converted into a digital signal, and then applied to the speed control means 1. According to the digital signal, the speed control means 1 controls the rotational speed of the spindle in the sewing machine body 14 in high speed mode. For instance, when the output voltage of the variable resistor 2 is 0 V, the rotational speed of the spindle of the sewing machine body 14 is controlled to 100 rpm; and when the output voltage of the variable resistor 2 is 5 V, the rotational speed of the spindle of the sewing machine body 14 is controlled to 4000 rpm. That is, the rotational speed in the high speed mode can be varied in the range of from 100 rpm to 4000 rpm by changing the output voltage of the variable resistor 2 in the range of 0 to 5 V.
In the low speed, positioning and thread cutting modes, similarly as in the above-described high speed mode, the variable resistors 3, 4 and 5 are changed in resistance and the analog switch 6 is operated separately according to these operations by the speed control means, so that the sewing speed of the sewing machine body 14 is suitably controlled.
In the conventional sewing machine speed control device thus constructed, the resistor 2 through 5 are operated to determine the sewing speed of the sewing machine body which is used in high speed, low speed, positioning or thread cutting mode. Therefore, in the case where different sewing machine bodies are used and they are driven at different speeds separately according high speed, low speed, positioning and thread-cutting operations for each sewing machine body, it is necessary to adjust the resistances of the variable resistors and change the sewing speeds for each sewing machine. However, this operation is rather troublesome.